1. Field of the Invention
The present invention relates to a method of manufacturing a bobbin for industrial use which has a flange made of synthetic resin on an end of a winding shaft made of nonferrous metal alloy. The invention is primarily concerned with the art of winding a filamentary material in the textile industry (IPC B65H54/00).
2. Description of the Prior Art
Bobbins for industrial use which comprise a winding shaft of nonferrous metal alloy and flanges of synthetic resin mounted respectively on both ends of the winding shaft, are presently commercially available in Japan, the U.S.A., and European countries. According to the method normally carried out for manufacturing such bobbins, a flange is formed by insert molding on an end of a winding shaft, for example, the end of the winding to which a spindle will be attached for rotating the bobbin during use. This manufacturing process is preferred since it is easy to insert a metal shaft into the bore in the winding shaft to enable the latter to withstanding the molding pressure during the molding step, and also it is easy to remove the metal shaft out of the bore in the winding shaft away from the formed flange. Therefore, no problem exists in forming a flange of synthetic resin on one end of a winding shaft in bobbins for industrial use. However, it has not been practiced to mold another flange on the other end of the winding shaft though the addition of such a flange is considered preferable. During the molding operation, the winding shaft could not withstand the molding pressure unless a metal shaft were inserted in the bore in the winding shaft. The metal shaft, however, could not be pulled out through the flange molded earlier on the end of the winding shaft after the additional flange has been formed. The first molded on the winding shaft has an axial hole having a diameter of smaller than the inside diameter of the metal shaft. As a consequence, the additional flange could not be molded on the other end of the winding shaft unless the metal shaft were inserted into the winding shaft bore from the other end thereof and then pulled out through the same other end after the flange has been formed thereon. For the above reason, it has been impossible to form a flange on the end of the winding shaft opposite to the flange end thereof.
To cope with the foregoing difficulty, the manufacturing process now practiced in the art includes the steps of forming a flange on one end of a winding shaft by insert molding, then forming screw threads on the inner peripheral surface of the bore in the winding shaft at the other end, forming screw threads on a projection on a separately formed flange having a T-shaped cross section, then threading the externally threaded flange projection into the internally threaded bore in the winding shaft to thereby fasten the flange to the other end of the winding shaft. Since the bore in the winding shaft has to be internally threaded, the winding shaft must be of an increased thickness, and hence is heavy. This manufacturing process requires the additional steps of threading the flange projection and the winding shaft bore. As the flange cannot sufficiently be fastened to the winding shaft merely by threading the flange projection into the winding bore, it has been necessary to insert a screw threadedly into the winding shaft from the outer side of the flange. The fastened flange tends to become loose and has to be frequently re-tightened during use because the flange is only mechanically fixed to the winding shaft. Therefore, the bobbin cannot be used for a prolonged period of time.
To solve the above disadvantages, the following manufacturing process has been practiced by some manufacturers in Japan: An end of a winding shaft is disposed in a molding die having a cavity shaped complementarily to a flange to be formed, and a melted mass of resin material is injected into the die cavity to mold the flange around the winding shaft end. The molding die has one or two gates positioned closely to the cavity and through which the resin material flows into the cavity. Since the melted resin is injected through the one or two gates into the cavity, the molded flange has a weld line and suffers molecular orientations at different densities of the resin material due to different injection pressures and different speeds at which the flange is cooled. With such a weld line and molecular orientations at different densities, the bobbin will be warped or vibrated and have increased out-of-roundness as the bobbin is rotated at a high speed such as a few thousands r.p.m. in operation, with the results that the bobbin will be out of balance and lowered in mechanical strength during high-speed rotation. It has been found that the mechanical strength of the portion of the flange which suffers the weld line is reduced 20% to 40% (Japanese Laid-Open Utility Model Publication No. 54-106321 published on July 26, 1979).
Japanese Utility Model Publication No. 45-13382 published on June 9, 1970 discloses a bobbin including a winding shaft of nonferrous metal having opposite ends bent at a right angle and a cylindrical bushing of synthetic resin inserted in the bore in the winding shaft. The bushing has an outer end projecting slightly from the end of the winding shaft and is joined to the winding shaft by an adhesive. A flange is injection-molded of synthetic resin around the projecting end of the bushing and the bent end of the winding shaft, thus providing an integral construction of the bushing, the bent end of the winding shaft, and the flange. With the disclosed bobbin arrangement, however, the bushing and the winding shaft are of different materials and bonded together. As the different materials are relatively poor in their being securely bonded together, the bonding strength tends to become weak in use over a period of time and thus the bushing and the winding shaft finally wobble relatively to each other, whereupon the bobbin can no longer be put to use. The flange molded around the projecting end of the bushing and the bent end of the winding shaft is, however, not joined sufficiently, and is frequently brought out of balance during high-speed rotation.
In the prior art bobbin disclosed in the latter publication, the end of the winding shaft or barrel of nonferrous metal for winding a filamentary material is bent into the flange around the bushing. In the bobbin shown in the former publication, the winding shaft has an end shaped into corrugated cross section including radially outward ridges before the flange is molded on the end of the winding shaft. Since the bent or corrugated end portion is formed of nonferrous metal, it cannot have a mechanical strength large enough to withstand the force acting axially on the flange from an inner side toward an outer side thereof due to the stress from the filamentary material wound on the winding shaft, even if the winding shaft is tempered or quenched.